Aluminum alloy and process for producing the same

ABSTRACT

A aluminum alloy in the form of bulk includes an aluminum matrix and carbon particles having an average particle size of 100 nm or less and dispersed in the aluminum matrix in an amount of 1 to 40 atomic % with respect to the total atoms constituting the aluminum alloy. The aluminum alloy is produced by preparing a raw material comprising aluminum and carbon as components and forming an aluminum alloy by inserting the raw material into a cavity formed by a set of dies and applying repeatedly plastic deformation to the raw material while maintaining the temperature of the raw material in the range of from 100 to 400° C.

BACKGROUND OF THE INVENTION

The present invention relates to an aluminum alloy having excellentproperties such as high strength, hardness, high modulus, low thermalexpansion coefficient, high heat resistance, and high wear resistance,and which is widely applicable to various industrial fields such as ofautomobiles, aircraft, electric appliances, and the like. The presentinvention also relates to a process for producing the same.

DESCRIPTION OF THE RELATED ARTS

An aluminum alloy has a high specific strength and various otherexcellent properties such as high strength, hardness, high thermalresistance, and high wear resistance, and is widely used in the field ofautomobiles, aircraft, electric appliances, etc. Particularly, it isexpected to exhibit excellent performance when used in rapid movingparts. For this reason, active study has been made on the productionmethods such as rapid cooling and mechanical alloying.

However, the application field of the products obtained by rapid coolingor mechanical alloying is limited, because they are in the form of apowder consisting of particles from several to several tens ofmicrometers (μm), or a ribbon about 20 μm in thickness. Accordingly, thepowder must be consolidated before using it as a component. In general,it is subjected to canning extrusion, HIP (hot isostatic pressing)process, etc., in the temperature range of from 400° to 550° C. under anon-oxidizing atmosphere. However, when subjected to such processes, theamorphous phase or the non-equilibrium phase undergoes crystallizationor equilibration because of the high temperature to provide, in general,a crystallized alloy. In addition, the dispersed particles precipitatedare flocculated to become coarse particles, so that the strength ofparticles declines. Furthermore, when a product is produced by canningextrusion effected at a low temperature, there is another problemconcerning inferior strength due to insufficient bonding between theparticles.

An aluminum alloy can be produced by adding a graphite powder ofgraphite into an aluminum melt being stirred, and casting the resultingmelt thereafter. However, it is difficult to uniformly disperse thegraphite powder into the melt. Moreover, because graphite particles areas large as 1 to 30 μm in diameter and because they do not bond withaluminum, they tend to undergo spalling at the boundary with thealuminum alloy. In case of forcibly stirring and mixing a powder ofaluminum with a powder of graphite to effect mechanical alloying, alarge part of graphite undergoes reaction with aluminum to form aluminumcarbide. Thus, a large quantity of graphite, which is effective as alubricant, is lost from the resulting material. Furthermore, in case ofconsolidation of the mixed powder into a bulk material, it requirescanning extrusion and the like to be performed in a temperature range offrom 400° to 550° C. The remaining graphite then changes into relativelylarge aluminum carbide crystals which impair the strength of theresulting alloy. Moreover, age-hardening by the precipitation ofaluminum carbide is not expected to occur on the alloy. This is anotherdisadvantage of this process.

It can be seen from the foregoing that a material obtained byconventional processes such as rapid cooling or mechanical alloyingcomprises a non-equilibrium phase, etc., and it results in the form of apowder or a ribbon. Accordingly, a serious problem of processing thematerial into a shaped product by means of canning extrusion and thelike must be overcome. Thus, it is strongly demanded to develop aneconomical and easy process for producing an aluminum alloy, whichenables a bulk material containing a non-equilibrium phase and the likehaving superior properties such as high strength, hardness, high elasticmodulus, low thermal expansion coefficient, high heat resistance, andhigh wear resistance.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an aluminum alloy inthe form of bulk suitable for use in various industrial fields inclusiveof automobiles, aircraft, and electric appliances, the aluminum alloyhaving superior properties such as high strength, hardness, highmodulus, low thermal expansion coefficient, high heat resistance, andhigh wear resistance.

Another object of the present invention is to provide a process forproducing an aluminum alloy in the form of bulk having theaforementioned superior properties.

The present inventors studied the problems in detail to achieve theobjects above. The present invention is accomplished based on thefollowing findings.

A powder compact of a mixture obtained by mixing powders of purealuminum, carbon, and titanium was subjected repeatedly to a strongplastic deformation process by using a processing means whose directionof processing can be varied as shown in FIG. 1, and strained to a degreefar greater than a one which results by a conventional plasticdeformation. The inventors discovered at this experiment that theresulting material comprises structures of a non-equilibrium phaseinclusive of a super-saturated solid solution phase and others, withcarbon particles having a size in the order of nanometers (nm) beingfinely dispersed therein, and that a bulk material is obtainable bystrongly applying the plastic deformation alone. Furthermore, it hasbeen found that a material with a non-equilibrium phase consistingmainly of compounds finely dispersed therein, can be obtained by heatingthe bulk material to a temperature range of from 300° to 600° C. Thematerial obtained has a tensile strength of 70 kgf/mm² or higher, anelastic modulus of 130 GPa or higher, and a thermal expansioncoefficient of 15×10⁻⁶ /K or lower.

Throughout the specification, a bulk material represents a lump ofmaterial originally made of powders or particles, said powders orparticles being strongly bound to each other as in sintering or melting.The bulk material usually measures mm order or more.

In accordance with a first aspect of the present invention, there isprovided an aluminum alloy comprising an aluminum matrix and carbonparticles having an average particle size of 100 nm or less, said carbonparticles being dispersed in said matrix in an amount of 1 to 40 atomic% with respect to the total atoms constituting the aluminum alloy, saidaluminum alloy being in the form of bulk.

The aluminum alloy according to the first aspect of the presentinvention preferably comprises aluminum and carbon added therein in aquantity of from 1 to 40% by atomic. If carbon should account for 1% byatomic or less, only small effect would be exerted on producing a highstrength material improved in wear resistance. An addition of carbon ina quantity of 40% by atomic or higher embrittles the resulting material.Accordingly, carbon content falling out of the specified range is notpreferred. The carbon particles that are dispersed in the aluminummatrix are preferably 100 nm or less in average diameter. If carbonparticles should be larger than 100 nm in average diameter, the strengthand the heat resistance of the material would be impaired. From theviewpoint of achieving an aluminum alloy having high strength, hardness,high elastic modulus, low thermal expansion coefficient, high thermalresistance, and high wear resistance, particularly preferred are thecarbon particles whose size range from several to several tens ofnanometers in diameter. The aluminum alloy according to the first aspectof the present invention exhibits superior characteristics such as highstrength, hardness, high elastic modulus, low thermal expansioncoefficient, high thermal resistance, and high wear resistance becausecarbon particles 100 nm or less in average diameter are finely dispersedin the matrix. Particularly, when graphite is used as carbon, a materialhaving a low friction coefficient is obtained because graphite functionsas a lubricant.

In accordance with a second aspect of the present invention, there isprovided an aluminum alloy further comprising crystals of asuper-saturated solid solution phase and/or a non-equilibrium phasehaving an average crystal size of 100 nm or less, said crystals beingformed from a reaction between aluminum and at least one metal ornon-metal selected from the group consisting of elements of Groups 4a,5a, 6a, 7a, 8a of the periodic table, silicon and boron, dispersed insaid matrix in an amount of 0.5 to 20 atomic % with respect to the totalatoms constituting the aluminum alloy.

The aluminum alloy according to the second aspect of the presentinvention preferably contains aluminum, from 1 to 40% by atomic ofcarbon, and from 0.5 to 20% by atomic of at least one metal or non-metalselected. The content of carbon is limited to the range above because ofthe reason described above for the case of the aluminum alloy accordingto the first aspect of the invention. If metals and non-metals otherthan carbon should account for 0.5% atomic or less, they would have noeffect in reinforcing the material, whereas an addition thereof in acontent of 20% by atomic or more impairs the toughness of the material.Accordingly, a composition falling out of the specified range is notpreferred.

The average diameter of the carbon particles that are dispersed in thematrix of the aluminum alloy is limited by the same reason describedabove for the first aspect of the present invention. The averagediameter of the super-saturated solid solution phase and/or thenon-equilibrium phase of a compound and the like is limited to 100 nm orless because crystals of over 100 nm in average diameter would no longerbe effective as dispersed crystals. In particular, crystals from severalto several tens of nanometers are preferred from the viewpoint ofimproving the strength, because they have strong effect on suppressingslip dislocations. Furthermore, the super-saturated solid solution phaseand/or the non-equilibrium phase may contain therein carbon to form asolid solution. The characteristics of the resulting alloy such asstrength can be further improved by adding carbon to form a solidsolution.

The aluminum alloy according to the second aspect of the presentinvention exhibits superior characteristics such as high strength,hardness, high elastic modulus, low thermal expansion coefficient, highthermal resistance, and high wear resistance because carbon particlesand crystals of a super-saturated solid solution phase and/or anon-equilibrium phase generated through the reaction of aluminum and thealloy element, which are 100 nm or less in average diameter, are finelydispersed in the matrix. Particularly, when graphite is used as carbon,a material having a low friction coefficient results because graphitefunctions as a lubricant.

In accordance with a third aspect of the present invention, there isprovided an aluminum alloy wherein said carbon particles comprisecrystals of a non-equilibrium phase and/or an equilibrium phase mainlycomposed of aluminum carbide and having an average crystal size of 100nm or less. The aluminum alloy according to the third aspect of thepresent invention contains crystals of aluminum carbide finely dispersedin its matrix suppress slip dislocations, and exhibits superiorcharacteristics such as high strength, hardness, high elastic modulus,low thermal expansion coefficient, high thermal resistance, and highwear resistance.

In accordance with a fourth aspect of the present invention, there isprovided an aluminum alloy further comprising crystals of anon-equilibrium phase and/or an equilibrium phase having an averagecrystal size of 100 nm or less, said crystals being formed from areaction between aluminum and at least one metal or non-metal selectedfrom the group consisting of elements of Groups 4a, 5a, 6a, 7a, 8a ofthe periodic table, silicon and boron dispersed in said matrix in anamount of 0.5 to 20 atomic % with respect to the total atomsconstituting the aluminum alloy.

The aluminum alloy according to the fourth aspect of the presentinvention exhibits superior characteristics such as high strength,hardness, high elastic modulus, low thermal expansion coefficient, andhigh thermal resistance, because it contains crystals of anon-equilibrium phase and/or an equilibrium phase finely dispersed inthe matrix thereof.

In accordance with a fifth aspect of the present invention, there isprovided a process for producing an aluminum alloy, comprising the stepsof preparing a raw material comprising aluminum and carbon ascomponents, and forming an aluminum alloy in the form of bulk byinserting the raw material into a cavity formed by a set of dies andapplying repeatedly plastic deformation to the raw material with the setof dies while maintaining the temperature of the raw material in therange of from 100° to 400° C., the resulting aluminum alloy comprisingan aluminum matrix and carbon particles with an average particle size of100 nm or less dispersed in the aluminum matrix.

The process for producing an aluminum alloy according to the fifthaspect of the present invention is characterized in that a bulk materialhaving a shape similar to that of the final product is obtained byapplying repeated plastic deformation alone to finely disperse carbon inthe matrix. The reason why an aluminum matrix containing carbonparticles finely dispersed therein is obtainable is assumed as follows.

In case a shaped powder compact obtained from powders of aluminum andcarbon is subjected to repeated plastic deformation, for instance, theparticles of aluminum powder form a bond to each other throughdiffusion, but the particles of aluminum do not undergo bonding to thoseof carbon. Accordingly, carbon particles tend to be enclosed in thealuminum matrix. Thus, when the carbon particles enclosed in the matrixare subjected to plastic deformation, they undergo size-reduction toform unusually fine particles 100 nm or less in average diameter. Inparticular, when processing is applied in such a manner as changing thedirection of each processing, friction and crushing can be more easilyapplied to the powder. Otherwise, the processing can be repeated in onedirection only.

The process according to the fifth aspect of the present inventionprovides a bulk material by effecting it in a temperature range of from100° to 400° C. In the present process, high energy is applied to finelydivide carbon by friction and crushing, and the metallic powderparticles are strongly bonded to each other by applying high pressureand by taking advantage of the activated surface. The bonding of themetallic powder particles to each other occurs assumably by thediffusion of aluminum among the powder particles. The diffusion rate canbe increased most advantageously by elevating the process temperature.Moreover, from the viewpoint of minimizing the deformation resistance,the process is preferably effected at a higher temperature. However, toohigh a temperature accelerates the formation of an equilibrium phasesuch as aluminum carbide due to the diffusion reaction among the powderparticles. Accordingly, the process is preferably effected in atemperature range of from 100° to 400° C.

The process for producing an aluminum alloy in the form of bulkaccording to the fifth aspect of the present invention provides amaterial comprising an aluminum matrix finely dispersed therein carbonparticles of 100 nm or less in average diameter by a relatively simpleprocess of repeatedly applying plastic deformation to a powder compact.Thus, a material which exhibits superior characteristics such as highstrength, hardness, high elastic modulus, low thermal expansioncoefficient, high thermal resistance, high wear resistance and lowfriction coefficient can be realized. Furthermore, because the finalproduct is obtained in the form of bulk and not in the form of a powderor a ribbon, the process excludes danger which is found in theconventional process using a powder or saves a consolidation step ofpowders. Moreover, in case graphite is used as carbon, seizure ofaluminum in the forging dies can be considerably reduced. Accordingly,the process load can be reduced, and the processed product can be moreeasily dismounted from the dies.

In accordance with a sixth aspect of the present invention, there isprovided a process for producing an aluminum alloy, wherein said rawmaterial further comprises at least one member selected from the groupconsisting of elements of Groups 4a, 5a, 6a, 7a, 8a of the periodictable, silicon and boron as components, and said resulting aluminumalloy in the forming step further comprises crystals of asuper-saturated solid solution phase and/or a non-equilibrium phase withan average crystal size of 100 nm or less, said crystals being formedfrom said aluminum and said at least one member.

The process including repeated plastic deformation according to thesixth aspect of the present invention is characterized in that finelydispersed carbon particles and crystals of a super-saturated phaseand/or a non-equilibrium phase can be formed in the aluminum matrix byapplying repeated plastic deformation alone. Furthermore, the processprovides a shaped material easily applied to the final product. Asdescribed above, the fine dispersion of carbon and the formation of anon-equilibrium phase are realized by producing a material mainlycomposed of aluminum, carbon, and at least one metal or non-metalselected from the group consisting of elements of Groups 4a, 5a, 6a, 7a,8a, silicon, and boron and inserting the material into a set of dieswhile maintaining the gas atmosphere to be inert and the temperature ina range of from 100° to 400° C. The structure comprising the finelydispersed carbon is obtained by a similar effect described above in thefirst aspect of the present invention. The reason for the formation of anon-equilibrium phase is assumed as follows.

The process for producing an aluminum alloy according to the sixthaspect of the present invention comprises forming super-fine carbonparticles and crystals of a non-equilibrium phase having an averagediameter of several tens of nanometers or less, by taking advantage ofthe solid phase reaction phenomena similar to that employed in theconventional process of mechanical alloying. However, the processaccording to the present invention differs from the conventional ones inthe following points. Mechanical alloying process uses a ball mill toeffect milling for a duration of from 10 to 1,000 hours at a temperaturein the vicinity of the room temperature. In this manner, powderparticles are subjected to repeated friction, crushing, and aggregationto form an intergranular non-equilibrium phase. However, in thisprocess, a powder is obtained unexceptionably as the final product. Theresulting powder is active, but the surface activity is lost due to theslight absorption of atmospheric gas or to the formation of a compoundwhich occurs on the surface of the powder, or because of the longpassage of time after the formation of the active surface. Accordingly,in case of consolidation of the powder sample, the powder must be takenout from the ball mill, placed inside a vessel, and subjected to canningextrusion or HIP process at a high temperature in a range of form 450°to 600° C.

In contrast to the conventional process, the process according to thesixth aspect of the present invention comprises producing a bulkmaterial in a temperature range of from 100° to 400° C. by repeatedlyapplying high energy to effect plastic deformation. Accordingly, carbonparticles are finely divided by the friction and crushing appliedthereto, and a non-equilibrium phase is formed by allowing diffusionreaction to occur among the powder particles, while tightly bonding themetallic particles to each other by applying high pressure thereto andby taking advantage of the effect of the activated surface. The frictionamong the powder particles and crushing more readily occur on theparticles by changing the direction of each processing. Otherwise,processing may be effected in one direction. The effect of carbon on theformation of a non-equilibrium phase in case of using an aluminumpowder, a carbon powder, and a titanium powder, for instance, is thesame as that of the case using aluminum powder with carbon powder.However, in case of aluminum powder and titanium powder, the surface ofeach powder particle is activated by the friction and crushing that areexerted by the strong plastic deformation processing. Hence, diffusionis found to occur more readily among the aluminum and titanium powderparticles. In the next step of the process, the formation of an activesurface proceeds by applying further friction and crushing to the powderparticles. By repeatedly applying the plastic deformation process,bonding of aluminum to titanium occurs in the material by diffusion asto form a non-equilibrium phase. In case titanium is incorporated in alarge quantity, carbon is found to be finely dispersed in thenon-equilibrium matrix. In other words, a structure comprising anon-equilibrium phase with carbon particles finely dispersed therein ata size in the order of nanometers. In case the quantity of titanium issmall, on the other hand, a structure comprising carbon particles finelydispersed in the aluminum matrix is obtained.

A super-fine dispersion and a non-equilibrium phase are assumed to beformed by the diffusion which occurs among the powder particles and thelike under the application of a high energy. The diffusion rate can beincreased most advantageously by elevating the process temperature.Moreover, from the viewpoint of minimizing the deformation resistance,the process is preferably effected at a higher temperature. However, toohigh a temperature accelerates the formation of an equilibrium phasesuch as aluminum carbide due to the diffusion reaction among the powderparticles. In addition, even though once a non-equilibrium phase isformed, it turns into an equilibrium phase because the high temperatureis maintained. Accordingly, the process is preferably effected in atemperature range of from 100° to 400° C.

In case a cast article is used as the starting material, the repeatedplastic deformation process is applied to a stable phase dispersed inthe aluminum-alloy cast article in the form of relatively large carbonparticles or intermetallic compounds. Thus, while carbon is reduced tofine particles by crushing, friction and crushing are repeatedly appliedto each of the stable phases to obtain a structure with anon-equilibrium phase and a super-saturated solid solution phase finelydispersed therein.

The process for producing an aluminum alloy according to the sixthaspect of the present invention provides, by a relatively simple processof repeatedly applying plastic deformation to a powder compact, amaterial comprising an aluminum matrix with carbon particles of 100 nmor less in average diameter and crystals of a super-saturated phaseand/or a non-equilibrium phase finely dispersed therein. Thus, amaterial which exhibits superior characteristics such as high strength,hardness, high elastic modulus, low thermal expansion coefficient, highthermal resistance, high wear resistance and low friction coefficientcan be realized. Furthermore, because the final product is obtained inthe form of bulk and not in the form of a powder or a ribbon, theprocess excludes danger which is found in the conventional process usinga powder or saves a consolidation step of powders.

Moreover, in case graphite is used as carbon, seizure of aluminum in theforging dies can be considerably reduced. Accordingly, the process loadcan be reduced, and the processed product can be more easily dismountedfrom the dies.

The set of dies in the forming step may comprise one of the following:

(1) a plurality of trapezoidal punches disposed in the upper, lower,left and right positions to form a cavity surrounded by front portionsthereof by contacting each of said punches on side walls thereof;

(2) a die having a cylinder therein and a pair of punches inserted intothe cylinder, a cavity being formed in the cylinder and having anorifice with a small cross sectional area; and

(3) a die having a concave portion thereon and a punch opposing to theconcave portion, a cavity being formed between the concave portion andthe punch.

In accordance with a seventh aspect of the present invention, there isprovided a process for producing an aluminum alloy, further comprising aconversion step for forming a structure with a non-equilibrium phaseand/or an equilibrium phase mainly composed of a compound with aluminumdispersed in said aluminum matrix by heat treating said resultingaluminum alloy in a temperature range of from 300° to 600° C.

The process for producing an aluminum alloy according to the seventhaspect of the present invention is characterized in that a material issubjected to repeated plastic deformation to obtain a materialcomprising dispersed therein super-fine particles of carbon and crystalsof a non-equilibrium phase, and in that the resulting material issubjected to heat treatment to newly obtain a material finely dispersedtherein a non-equilibrium phase and/or an equilibrium phase. Thus isobtained a material having superior characteristics such as highstrength, hardness, high elastic modulus, low thermal expansioncoefficient, high thermal resistance, high wear resistance, and lowfriction coefficient.

The reason why an aluminum alloy having superior characteristics such ashigh strength is obtainable by the heat treatment above is assumed asfollows. By heattreating the aluminum alloy comprising thenon-equilibrium phase above at a temperature range of from 300° to 600°C., an alloy element diffused out from the super-saturated solidsolution and the like in the aluminum alloy matrix or an active elementfinely size-reduced to the order of nanometers form a structurecomprising finely dispersed therein a non-equilibrium phase or anequilibrium phase mainly composed of a compound with aluminum. Thus,strength and other characteristics can be improved. Moreover, thestrength remains without being impaired even in a temperature region ashigh as in a range of from 300° to 600° C.

According to a process for producing an aluminum alloy of the seventhaspect of the present invention, there is provided a relatively simpleprocess which comprises shaping relatively easily a material which isrelatively soft before heat-treating into the shape of a final product,and heat-treating the shaped material to obtain a high strength aluminumalloy material comprising finely dispersed therein a non-equilibriumphase and/or an equilibrium phase mainly composed of a high strengthcompound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are diagrams showing process steps for effectingrepeated processing (cross-shaped compression method);

FIGS. 2A, 2B, and 2C are diagrams showing other process steps foreffecting repeated processing (closed cross-shaped compression method);

FIG. 3 is a diagram showing a process step for effecting repeatedprocessing (extrusion method);

FIGS. 4A, 4B, 4C, 4D, 4E, and 4F are graphs showing the results of X-raydiffraction performed on a starting material, a forged sample subjectedto repeated processing (cross-shaped compression method) in Example 1,and samples each maintained at a temperature of 300° C., 400° C., 500°C., and 600° C.;

FIGS. 5A, 5B, 5C, and 5D are graphs showing the results of X-raydiffraction performed on a starting material and samples subjected torepeated processing (cross-shaped compression method) in Example 1 fordifferent repetition cycles of processing;

FIG. 6 is a graph showing the relation between the temperature and thehardness (Hv) of the samples subjected to repeated processing(cross-shaped compression method) in Examples 1, 4, 6, etc.;

FIGS. 7A, 7B, 7C, 7D, 7E, and 7F are graphs showing the results of X-raydiffraction performed on a starting material, a forged sample subjectedto repeated processing (cross-shaped compression method) in Example 4,and samples each maintained at a temperature of 300° C., 400° C., 500°C., and 600° C.; and

FIG. 8 is a graph which gives the elastic modulus and the thermalexpansion coefficient for various types of aluminum materials in Example4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in further detail for each of thealuminum alloys and the processes for producing the same, by referringto each of the first to the seventh aspect of the present inventionabove.

In the detailed description below, the elements to be mixed in thepowder of aluminum, etc., which constitutes the material to beprocessed, include carbon in fine particles as an essential one. Thereis no particular restriction concerning the type of carbon to be used inthe present invention, and commonly used graphite and amorphous carboncan be employed. Furthermore, an element which readily forms anon-equilibrium phase such as a super-saturated solid solution or anintermetallic compound must be added to the powder system. Preferably,when the material is subjected to the final step of heat treatment, thiselement allows a non-equilibrium phase or a stable phase based on ametallic compound to precipitate as a fine dispersion in the aluminumalloy matrix. Moreover, the element preferably remains without beingaggregated even at high temperatures, and enables the formation of fineparticle precipitates. Preferred from these points of view is at leastone metal or non-metal selected from the group consisting of elements ofGroups 4a, 5a, 6a, 7a, 8a, silicon, and boron.

The amount of carbon to be added is preferably in a range of from 1 to40% by atomic. If carbon is added in an amount of 1% by atomic or less,only small effect would be exerted on producing a high strength materialimproved in wear resistance. An addition of carbon in an amount of 40%by atomic or higher embrittles the resulting material. The metals ornon-metals other than carbon is preferably added at an amount of from0.5 to 20% by atomic. If metals and non-metals other than carbon shouldaccount for 0.5% atomic or less, they would have no effect inreinforcing the material, whereas an addition thereof at a content of20% by atomic or more impairs the toughness of the material.

There is no particular restriction concerning the morphology of thematerial to be processed. More specifically, for example, a mixed powdercomprising aluminum and carbon; a mixed powder comprising aluminum,carbon, and at least one element selected from the group consisting ofelements of Groups 4a, 5a, 6a, 7a, 8a, silicon, and boron; and a powdercompact or a cast article obtained from the mixed powders above; can beused without any problem.

Specifically in the present invention, carbon particles andnon-equilibrium phases and the like dispersed in the matrix of aluminumand the like are preferably 100 nm or less in size. From the viewpointof increasing the strength of the bulk material, more preferably, theyare from several to several tens of nanometers in size. Finely dispersedcarbon particles and non-equilibrium phase can be formed by: (A)repeatedly applying plastic deformation for the crushing, the formationof new surfaces, and the diffusion of elements of the aluminum powder,the powder of an additive element, and the various types of phasespresent in the aluminum alloy; and (B) heating the material at atemperature not lower than 100° C. but not higher than the temperatureat which an equilibrium phase is formed, i.e., 400° C., therebyfacilitating the plastic deformation and diffusion. The process can beperformed at a temperature falling out of the range defined above,however, at an expense of low diffusion rate.

Plastic deformation must be applied regardless of what type of materialis used, for example, in case of using a mixed powder of the startingelements, a powder compact obtained by compressing the thus obtainedmixed powder, or a cast article of an aluminum alloy obtained by meltingprocess and the like and containing dispersed therein a stable phase. Byapplying plastic deformation, each of the phases is subjected torepeated friction and crushing with each other to obtain an activatedinterface. Furthermore, a sufficiently high draft and load must beapplied to bond the particles by diffusion. Under such an intenseplastic deformation, diffusion and consolidation occur at a part of thesurface brought into contact with each other to confine carbon and thelike to a limited area. To newly subject the thus enclosed carbon andthe like to crushing in the subsequent process step, and to thereby forman activated surface, the plastic deformation is repeated at leastseveral tens of cycles. The processing stress is applied at leastequivalent to the yield strength of the aluminum alloy, i.e., at 20kg/mm² or higher. Preferably, by taking the friction which occurs on thesliding plane of the mold and the damage of the mold into consideration,the processing is effected under a stress of from 60 to 200 kg/mm².

Repeated plastic deformation processing can be effected, for example, bymethods described below:

(1) Cross-shaped compression

This method employs a set of dies with movable punches arranged in theperpendicular and the horizontal directions equipped in a processingmachine commonly employed in pressing and the like. More specifically,the material to be processed is placed in the center portion, and iscompressed by a punch 1 from the direction A. The material iscompressed, but because a punch 2 is provided movable, a part of thematerial is extruded in the direction perpendicular to the direction inwhich the load is applied. Then, by operating the punch 2, the materialis compressed by applying a load from the direction B. Processingproceeds in this manner by repeating this operation sequence. It can beseen that one of the punches directly drives the other. Accordingly, thesample can be greatly deformed. A disadvantage of this method is thatthe material to be processed may be subject to cracking as the volume ofthe cavity in the dies changes. This disadvantage can be overcome byusing an equipment of a closed type, as shown in FIGS. 2A to 2C, whichkeeps the volume of the cavity almost constant. In the latter case, itis desirable to provide a mechanism which interlinks the advancingpunches with the retreating punches.

(2) Extrusion:

This method employs a die as shown in FIG. 3. The material to beprocessed is placed between the two punches. As the punches reciprocate,the material to be processed is forced through the narrow orifice 31 upand down. When the upper punch 1 moves downward under load, the lowerpunch 11 also moves downward while keeping the confined volume of thecavity. Thus, the extruded material has its cross-sectional areaexpanded as large as that of a punch. This method permits effectiveplastic deformation without causing no cracking to the material to beprocessed, owing to the closed extrusion which keeps the volume of thecavity almost constant.

(3) Rotary forging:

This method employs a device consisting of a stationary die and a punchplaced above. The material to be processed is placed at the center ofthe die, and undergoes plastic deformation when a local pressing isapplied by rotating and vibrating the upper punch. Deformation per cycleis relatively small, but repeated plastic deformation can be easilyapplied. Moreover, materials of large size can be processed by thismethod because processing load can be minimized.

In addition to the fulfillment of the conditions (A) and (B) above, incase a powder material is used as the material to be processed, theprocess is preferably effected under an inert gas atmosphere to maintainthe surface of aluminum and the like clean. Under an inert gasatmosphere, diffusion between aluminum and the surface of various otherphases can be favorably effected. Even when the powder are crushed toform new active surfaces by plastic deformation, surface activity wouldbe lost if oxidation or nitridation occurs due to the atmospheric gas.Accordingly, to maintain the activity of the newly formed surface, theprocess is effected under high vacuum or in an inert gas atmosphere suchas of argon.

The present invention is described in further detail below referring topreferred embodiments.

EXAMPLE 1

A pure aluminum powder passed through a 350-mesh sieve and a powder ofpure graphite composed of particles about 1 μm in average diameter weremixed in the atomic ratio of 80:20, and after sufficiently mixing themixed powder, a powder compact 20×10×8.5 mm³ (length×width×height) insize was obtained therefrom by using a hydraulic press operated at apressure of about 1,000 kgf/cm². The powder compact weighed 3.7 g. Theresulting sample was placed and set in the center of the set of dies,and heated to 300° C. by setting the dies in an electric furnace whileflowing argon gas at a flow rate of from 1 to 3 l/min to preventoxidation. Then, after taking the entire dies out of the electricfurnace, the dies were set in a pressing machine equipped with amechanism which applies pressures from the upper and the lower side ofthe dies, and pressure was applied from the direction A shown in FIG. 1Bto compress the powder compact therein to a thickness of 2 mm. By thecompression operation, a part of the sample was found to be extruded inthe direction perpendicular to the direction A. Then, the dies wererotated by an angle of 90° to apply pressure thereto from direction B asshown in FIG. 1C until the sample was compressed to a thickness of 2 mm.This sequential operation was repeated for 120 cycles. The maximumcompression load in the initial stage of the process was about 15 ton,but it was found to increase up to 20 ton after performing the operationfor 120 cycles.

The dies were disintegrated to take the sample out from the dies. Aslight crack was found to generate on the part of surface of the thusobtained sample, but the powder particles were found to be tightlybonded with each other to provide a material bulk. On observing thecross section of the sample under a microscope, no cracks nor inclusionsand the like was observed.

The sample was subjected to X-ray diffraction to obtain a pattern asshown in FIG. 4B. FIG. 4A shows the presence of graphite in the startingmaterial, but graphite is no longer identified in the resulting productas shown in FIG. 4B. Under transmission microscope, graphite particlesfrom 5 to 10 nm in average diameter were found to be dispersed in thealuminum matrix. By analyzing the results, it was found that the samplecontained fine graphite particles unidentifiable by X-ray diffraction.Considering conventional casting processes in which graphite isincorporated in an aluminum matrix as particles about 1 to 20 μm indiameter, the present example enables extremely fine graphite particlesfrom 5 to 10 nm in average diameter.

FIGS. 5A to 5D show the influence of repeated processing on the diameterof graphite. The starting material used for the experiment shown in FIG.5A comprises graphite particles about 1 μm in diameter. FIG. 5B showsthe change on the X-ray diffraction pattern on increasing the repetitioncycles. It can be seen that there is no distinguished change in theX-ray pattern after 40 cycles of plastic deformation processing, but byanalyzing the broadening of the diffraction pattern, the averagediameter of the particles of graphite was found to be about several tensof nanometers. After repeating processing for 80 cycles, as shown inFIG. 5C, the graphite particles were found to be 12 nm in diameter. Asshown in FIG. 5D, after repeating processing for 120 cycles, sizereduction of the graphite particles proceeded rapidly as to yieldparticles having an average diameter of several nanometers. A part ofthe powder compact subjected to compression was heat-treated (agingtreatment) for 1 hour in argon gas flow while maintaining thetemperature at 300° C., 400° C., 500° C., and 600° C. The X-raydiffraction patterns of each of the samples are given in FIGS. 4C, 4D,4E, and 4F, and the results obtained by measuring Vicker's hardness (Hv)at room temperature are given in FIG. 6.

The sample subjected to repeated processing yields a structurecomprising finely dispersed graphite particles as shown in FIG. 4B. Thehardness of the sample was found to be Hv 100. However, by subjectingthe sample to aging treatment, an aluminum carbide (Al₄ C₃)-like phasewas found to develop at about 500° C., which converts into anequilibrium phase Al₄ C₃ in the vicinity of a higher temperature of 600°C. At the same time, a maximum hardness of Hv 220 was obtained. The agehardening characteristics can be observed not only on graphite, but alsoon amorphous carbon.

As described in the foregoing, the process for producing an aluminumalloy according to the present invention comprises repeatedly processingthe material, and it provides a super-fine structure of graphite, whichhas been hardly achieved by a conventional process. Furthermore, a bulkmaterial further improved in hardness can be obtained by subjecting thematerial to aging treatment.

EXAMPLE 2

A pure aluminum powder passed through a 350-mesh sieve and a powder ofpure graphite composed of particles about 1 μm in average diameter weremixed in the atomic ratio of 95:5, and after sufficiently mixing themixed powder, the sample was subjected to repeated processing for 120cycles in a set of dies whose temperature was set at 300° C. in the samemanner as in Example 1 to obtain a powder compact of 2 mm in thickness.A slight crack was found to generate on the surface of the thus obtainedsample, but the powder particles were found to be tightly bonded witheach other to provide a bulk material.

The sample was subjected to X-ray diffraction to obtain a patternsimilar to that of FIG. 4B, from which graphite cannot be identified.Thus, the aluminum alloy sample was found to be an alloy containingdispersed therein super-fine graphite particles 10 nm or less in averagediameter.

EXAMPLE 3

A pure aluminum powder passed through a 350-mesh sieve and a powder ofpure graphite composed of particles about 1 μm in average diameter weremixed in the atomic ratio of 60:40, and after sufficiently mixing themixed powder, the sample was subjected to repeated processing for 120cycles in a set of dies whose temperature was set at 300° C. in the samemanner as in Example 1 to obtain a powder compact of 2 mm in thickness.

The sample thus obtained was subjected to X-ray diffraction to obtain apattern comprising peaks assigned to graphite and broad ones forgraphite. The diameter of the crystals determined from the broadening ofthe X-ray diffraction pattern was about 15 nm. Al₄ Cl₃ was found toprecipitate by heating the sample to 600° C.

EXAMPLE 4

A mixed powder containing 10 atomic % each of graphite and titanium withrespect to aluminum was mixed, and was subjected to compressionprocessing. Aluminum and graphite powders were the same type as thoseused in Example 1. Titanium was in the form of powder passed through a350-mesh sieve. The mixed powder sample was placed inside a set of diesshown in FIGS. 1A to 1C, and was maintained at a temperature of 300° C.in the same manner as in Example 1, while repeatedly applyingcompression deformation to the sample for 120 cycles. The sample thusobtained from the disintegrated dies was found to be in the form of bulkhaving no cracks and powder particles sufficiently bonded to each other.

The X-ray diffraction patterns of each of the samples heated at 300° C.,400° C., 500° C. and 600° C. are given in FIGS. 7C, 7D, 7E, and 7F, andthe results obtained by measuring Vicker's hardness (Hv) at roomtemperature are given in FIG. 6. The sample subjected to repeatedforging processing as shown in FIG. 7B was found to comprise asuper-saturated solid solution phase of aluminum containing a phase ofpure aluminum and titanium as solid solution, and graphite particlesfinely dispersed therein. The hardness thereof was found to be Hv 122.By heating the sample to 500° C. to perform aging treatment, anon-equilibrium phase not identified in the equilibrium diagrams at roomtemperature was found to develop, and the hardness increased to Hv 210.The tensile strength at room temperature after repeated processing wasfound to be 30 kgf/mm², but it was found to be greatly improved to 85kgf/mm² by performing aging treatment at 500° C.

Then, a bulk material containing 10% atomic % of graphite with respectto aluminum (Al-10at%C) was prepared in the same manner as in Example 1.The bulk material thus obtained and a bulk material obtained by adding10 atomic % each of graphite and titanium with respect to aluminumprepared in Example 4 (Al-10at%C-10at%Ti), were compared withcomparative materials, i.e., commercially available pure aluminum andDuralumin (A2024), in terms of elastic modulus and thermal expansioncoefficient. With respect to the elastic modulus, samples of 1×2×1 mm³in size were prepared to from each material by cutting processing, andwere measured by piezoelectric composite bar method. The thermalexpansion coefficient was measured on the same samples at a heating rateof 5° C./min to obtain the average thermal expansion coefficient over atemperature range of from 50° to 200° C.

FIG. 8 shows the comparison of elastic modulus and thermal expansioncoefficient of each sample. Pure aluminum and a high strength aluminumalloy known as Duralumin (A2024) yield well comparable results forelastic moduli, which are 70 GPa and 74 GPa, respectively, and forthermal expansion coefficient, which are 24.4×10⁻⁶ /K and 23.5×10⁻⁶ /K,respectively. However, Al-10at%C-10%Ti yields an elastic modulus of 138GPa, a value twice as large as that of pure aluminum, and a thermalexpansion coefficient of 14.7×10⁻⁶ /K, a value reduced to about 60% ofthat of pure aluminum. In case of Al-10at%C, the elastic modulus wasfound to be increased by about 10%, and the thermal expansioncoefficient was found to be reduced by about 17% as compared with thoseof pure aluminum. Thus, the material in the form of bulk according tothe present invention yields an elastic modulus equivalent to that oftitanium and a thermal expansion coefficient equivalent to that ofsteel, which are far improved as compared with the conventional aluminumalloys. Thus, the aluminum alloy according to the present invention canbe used in the parts of precision equipments and electric componentssuch as a needle valve for use in fuel injection nozzles.

The elastic modulus and the thermal expansion coefficient of the othermaterials in the form of bulk according to the present invention werestudied to obtain similar favorable results.

EXAMPLE 5

A sample of 15 mm in diameter and 25 mm in height was prepared by usinga powder of the same composition as that used in Example 1. Then, a setof extrusion dies as shown in FIG. 3 was prepared. After applyinggraphite to the inner surface and the sliding portion of the die whichis to be brought into contact with the sample, the sample was placedinside the extrusion dies, and was maintained at a temperature of 300°C. Upon reaching the predetermined temperature, a load of 18 ton wasapplied from one punch by using a hydraulic press, and the dies wereturned upside down to apply a load from the other punch. The repeatedcompression processing was performed in this manner for 60 cycles.

The aluminum alloy thus obtained by extrusion was found to be completelyconsolidated, and was in the form of bulk free of cracks. Pure aluminumalone was identified by X-ray diffraction, and no graphite was observed.Thus, graphite is assumably present in the form of dispersed super-fineparticle from 5 to 10 nm in average diameter.

EXAMPLE 6

A pure aluminum powder passed through a 350-mesh sieve, a pure graphitepowder comprising particles about 1 μm in diameter, and a powder of pureiron passed through a 350-mesh sieve were mixed in the atomic ratio of80:10:10, and after sufficiently mixing the mixed powder, the sample wassubjected to repeated compression processing for 120 cycles in a set ofdies whose temperature was set at 300° C. in the same manner as inExample 1 to obtain a powder compact of 2 mm in thickness. The dies weredisintegrated, and the sample was thus taken out of the dies. A slightcrack was found to generate on the surface of the thus obtained sample,but the powder particles were found to be tightly bonded to each otherto provide a bulk material.

The sample was then subjected to X-ray diffraction analysis. One of thestarting materials, graphite, was not identified on the X-raydiffraction pattern. From magnetic analysis, the content of pure ironwas found to be low. Thus, it was found that the aluminum alloy of thesample consists of a structure mainly composed of an alloy containingiron in the form of solid solution and graphite dispersed therein asfine particles. The resulting alloy was heated at the temperatures 300°C., 400° C., 500° C., and 600° C. for an hour each to find fine crystalsof aluminum compound (Al₆ Fe) as a non-equilibrium phase and those of anequilibrium phase (Al₃ Fe), thereby being precipitated from the alloy.The hardness of the alloy was found to increase to Hv 385 from theinitial Hv 170 by heating as shown in FIG. 6. The same effect wasobserved in case silicon was used in the place of pure iron.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. An aluminum alloy comprising:an aluminumcontaining matrix; and carbon containing particles having an averageparticle size of 100 nm or less, said carbon containing particles beingdispersed in said matrix in an amount of 1 to 40 atomic % with respectto the total atoms constituting the aluminum alloy, said aluminum alloybeing in bulk form.
 2. An aluminum alloy as claimed in claim 1, furthercomprising crystals of a super-saturated solid solution phase and/or anon-equilibrium phase having an average crystal size of 100 nm or less,said crystals being formed from a reaction between aluminum and at leastone metal selected from the group consisting of elements of Groups 4a,5a, 6a, 7a and 8a of the periodic table, dispersed in said matrix in anamount of 0.5 to 20 atomic % with respect to the total atomsconstituting the aluminum alloy.
 3. An aluminum alloy as claimed inclaim 1, further comprising crystals of a super-saturated solid solutionphase and/or a non-equilibrium phase having an average crystal size of100 nm or less, said crystals being formed from a reaction betweenaluminum and at least one non-metal selected from the group consistingof silicon and boron, dispersed in said matrix in an amount of 0.5 to 20atomic % with respect to the total atoms constituting the aluminumalloy.
 4. An aluminum alloy as claimed in claim 1, wherein said carboncontaining particles comprise crystals of a non-equilibrium phase and/oran equilibrium phase mainly composed of aluminum carbide and having anaverage crystal size of 100 nm or less.
 5. An aluminum alloy as claimedin claim 4, further comprising crystals of a non-equilibrium phaseand/or an equilibrium phase having an average crystal size of 100 nm orless, said crystals being formed from a reaction between aluminum and atleast one metal selected from the group consisting of elements of Groups4a, 5a, 6a, 7a and 8a of the periodic table, dispersed in said matrix inan amount of 0.5 to 20 atomic % with respect to the total atomsconstituting the aluminum alloy.
 6. An aluminum alloy as claimed inclaim 4, further comprising crystals of a non-equilibrium phase and/oran equilibrium phase having an average crystal size of 100 nm or less,said crystals being formed from a reaction between aluminum and at leastone non-metal selected from the group consisting of silicon and boron,dispersed in said matrix in an amount of 0.5 to 20 atomic % with respectto the total atoms constituting the aluminum alloy.
 7. An aluminum alloyas claimed in claim 1, wherein said carbon containing particles comprisealuminum carbide.
 8. An aluminum alloy as claimed in claim 1, whereinsaid aluminum containing matrix consists of an aluminum alloy.
 9. Analuminum alloy as claimed in claim 1, wherein said carbon containingparticles consist of graphite or amorphous carbon.
 10. A process forproducing an aluminum alloy, comprising the steps of:preparing a rawmaterial comprising aluminum and carbon as components; and forming analuminum alloy in bulk form by inserting the raw material into a cavityformed by a set of dies and repeatedly applying plastic deformation tothe raw material with the set of dies while maintaining the temperatureof the raw material in the range of from 100° to 400° C., the resultingaluminum alloy comprising an aluminum containing matrix and carboncontaining particles with an average particle size of 100 nm or lessdispersed in the matrix.
 11. A process for producing an aluminum alloyas claimed in claim 10, wherein said raw material in the preparing stepfurther comprises at least one member selected from the group consistingof elements of Groups 4a, 5a, 6a, 7a and 8a of the periodic table, ascomponents; and said resulting aluminum alloy in the forming stepfurther comprises crystals of a super-saturated solid solution phaseand/or a non-equilibrium phase with an average crystal size of 100 nm orless, said crystals being formed from said aluminum and said at leastone member.
 12. A process for producing an aluminum alloy as claimed inclaim 10, wherein said raw material in the preparing step furthercomprises at least one non-metal selected from the group consisting ofsilicon and boron, as components; and said resulting aluminum alloy inthe forming step further comprises crystals of a super-saturated solidsolution phase and/or a non-equilibrium phase with an average crystalsize of 100 nm or less, said crystals being formed from said aluminumand said at least one member.
 13. A process for producing an aluminumalloy as claimed in claim 10, 11, or 12, wherein the preparing step isperformed by compressing powders of said components or casting a melt ofsaid components.
 14. A process for producing an aluminum alloy asclaimed in claim 10, 11 or 12, wherein said plastic deformation isapplied to the raw material at a stress of 20 kg/mm² or higher.
 15. Aprocess for producing an aluminum alloy as claimed in claim 10, 11 or12, wherein the set of dies comprises a plurality of trapezoidal punchesdisposed in the upper, lower, left and right positions to form a cavitysurrounded by front portions thereof by contacting each of said puncheson side walls thereof, and said forming step is performed by placingsaid raw material in the cavity and by compressing repeatedly the rawmaterial in alternate directions between said upper and lower punches orsaid left and right punches in such a manner that punches not workingstand free so as not to inhibit plastic deformation.
 16. A process forproducing an aluminum alloy as claimed in claim 10, 11, or 12, whereinthe set of dies comprises a die having a cylinder therein and a pair ofpunches inserted into the cylinder, and said cavity is formed in thecylinder and has an orifice with a small cross sectional area, and theforming step is performed by placing the raw material in said cavity andby extruding said raw material through said orifice with one of thepunches while the other of the punches moves in such a manner that thevolume of the cavity is maintained constant.
 17. A process for producingan aluminum alloy as claimed in claim 10, 11 or 12, further comprising aconversion step for forming a structure with a non-equilibrium phaseand/or an equilibrium phase mainly composed of a compound with aluminumdispersed in said aluminum containing matrix by heat treating saidresulting aluminum alloy in a temperature range of from 300° to 600° C.18. A product comprising an aluminum containing matrix and carboncontaining particles produced by a process comprising:preparing a rawmaterial comprising aluminum and carbon; inserting the raw material intoa cavity formed by a set of dies, and repeatedly applying plasticdeformation to the raw material with the set of dies, while maintainingthe temperature of the raw material in the range of from 100° to 400°C., resulting in an alloy comprising the aluminum containing matrix andthe carbon containing particles dispersed in the matrix, the particleshaving an average size of 100 nm or less.